When a user equipment (UE) is roaming among multiple cells using different radio access technologies (RATs, such as for example GSM, UTRAN and E-UTRAN/LTE) with idle state signaling reduction (ISR), the UE needs to perform some type of location registration to indicate the UE's availability to the relevant network cell, which is necessary for the cell to be able to page the UE such as for an incoming call.
There are various specific types for this location registration. Location registration is the more generic term which for example can be satisfied via a Tracking Area Update (TAU), a Routing Area Update (RAU), a combined TAU and RAU in the packet switched (PS) domain, and via Location Updating (LU) in the circuit switched (CS) domain. Location registration of a UE moving between different Location Areas (LA), Routing Areas (RA) and Tracking areas (TA) is therefore necessary, and even periodic location updating is necessary when the UE is not actively mobile. These periodic location registrations are controlled by dedicated timers; timer T3212 for the periodic LU, timer T3312 for the periodic RAU, and timer T3412 for the periodic TAU. Any of these may be generalized as a periodic location registration (LR) timer.
A problem arises if the UE is unable to perform its specific location registration. This may occur for example if the UEs access to network is barred, or if the network rejects the request from the UE with its back-off timer already set. In either case the UE is not allowed to perform its periodic location registration at the expiry of the relevant periodic update timer. When for whatever reason the UE fails to perform its periodic location registration on time, then once the relevant periodic location registration timer and implicit detach timer expires in the network, the core network entity will consider the UE no longer reachable and the core network (CN) will implicitly detach the UE. At this point, the UE becomes un-reachable for mobile terminating (MT) calls.
The general concept is shown at the timing diagram of FIG. 1. While its controlling LR timer is running the UE attempts to connect to the network to report its location registration. The timer expires without a LR update from this UE. The network also runs a similar timer for this particular UE and recognizes that it has received no location registration when that timer expires. Once the core network recognizes this it assumes the UE is no longer available (e.g., it assumes the UE has powered down or is not within range of any cell), and from that point onwards if there is an incoming call to the UE the core network does not know how it should route the call. No cell attempts to page the UE because the time for the UE to update its location registration has passed without a periodic update from the UE.